the  rnrtomy  or  the  hehd-rnd  mouth-  parts 

or 

PL  ECO PT ERR 


% 


THE  ANATOMY  OF  THE  HEADLAND  MOUTH-PARTS  OF  PLECOPTERA 


BY 

GLADYS  HOKE 

B.  S.  Mississippi  State  College  for  Women,  1916 


THESIS 

Submitted  in  Partial  Fulfillment  of  the  Requirements  for  the 

Degree  of 

MASTER  OF  SCIENCE 
IN  ENTOMOLOGY 

IN 

THE  GRADUATE  SCHOOL 

OF  THE 

UNIVERSITY  OF  ILLINOIS 


1921 


Digitized  by  the  Internet  Archive 

in  2016 


https://archive.org/details/anatomyofheadmouOOhoke 


UNIVERSITY  OF  ILLINOIS 


THE  GRADUATE  SCHOOL 


June  4,  1921  i9i 


1 HEREBY  RECOMMEND  THAT  THE  THESIS  PREPARED  UNDER  MY 

SUPERVISION  BY Gladys  He  kg 

ENTITLED  The  Anatomy  of  the  Head  and  Mouth-p arts  of 


Plecoptera 


BE  ACCEPTED  AS  FULFILLING  THIS  PART  OF  THE  REQUIREMENTS  FOR 


THE  DEGREE  OF 


OH.  *r. 


In  Charge  of  Thesis 


Head  of  Department 


Recommendation  concurred  in* 


Committee 

on 

Final  Examination* 


*Required  for  doctor’s  degree  but  not  for  master’; 


■ 


, 


1 


C 0 1'j  T 3 II  T S 


Introduction 
Fixed  Parts  of  the  Head 
movable  Parts  of  the  Head 
Summary 

Explanation  of  Plates 


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23 

34 

37 


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INTRODUCTION 

The  students  of  the  various  groups  of  insects  are  seriously 
handicapped  in  their  studies  of  the  homology  of  structures  on  ac- 
count of  a lack  of  sufficient  accurate  morphological  data  on  relat- 
ed groups.  This  study  of  the  Plecoptera  is  an  attempt  to  increase 
the  present  knowledge  of  the  morphology  of  the  head  of  generalized 
insects  as  well  as  to  amplify  the  studies  of  the  order. 

The  stoneflies  are  usually  thought  of  as  a very  generalized 
group.  This  fact  has  "been  emphasized  by  both  Comstock  and  Cramptonc 
An  examination  of  the  heads  of  the  order  will  reveal  at  once 

the  presence  of  many  Orthopteran  characteristics.  Altho  this  is 
true  in  many  respects,  in  others  they  exhibit  striking  speciliza- 
tions.  Smith  (1917)  calls  attention  to  the  highly  specialized  con- 
dition of  the  genitalia;  Klapalek  (1909),  probably  the  most  import- 
ant of  the  European  students  of  the  group,  has  examined  with  con- 
siderable detail  the  genitalia  and  the  venation  of  the  wings; 
Koponen  (1917)  has  studied  in  some  detail  the  nymphs  and  adults 
of  the  species  of  Finland;  Tillyard  (1921)  points  out  specializa- 
tions in  the  wing  venation,  the  coriae  and  the  mouth-parts  of 
Australian  species.  Studies  of  the  morphology  of  the  head  have  been 
made  by  Schoenemund  (1912)  in  connection  with  the  morphology  of 
the  thorax  and  abdomen,  and  by  Comstock  and  Kochi  (1902)  who  des- 
cribed and  figured  the  head  of  a nymph  of  Pteronarcys  in  their 
study  of  the  skeleton  of  the  head  of  insects,  oo  far  as  i have  been 
able  to  ascertain,  no  work  ha3  been  done  on  the  morphology  of  the 
head  and  the  tentorium  of  a representative  series  of  the  genera 
of  the  order.  An  effort  has  been  made  to  study  as  comprehensive  a 
series  as  possible,  one  which  would  show  both  the  generalized  and 


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the  specialized  conditions  of  the  head- capsule,  the  tentorium  and 
the  mouth-parts. 

The  specimens  were  prepared  for  study  by  placing  in  vials  con- 
taining a two  per  cent  solution  of  potassium  hydroxide  and  by  heat- 
ing the  vials  in  a water-bath  from  fifteen  to  thirty  minutes.  The 
potash  was  removed  by  washing  the  specimens  in  distilled  water. 

The  specimens  were  studied  under  a binocular  microscope.  Occasion- 
ally staining  with  Gage’s  Saurfuchsin  was  necessary  and  in  a few 
instances  the  specimens  were  placed  in  lactic  acid  for  fifteen  to 
thirty  minutes  in  order  to  distend  wrinkled  or  collapsed  parts. 

The  use  of  an  ocular  micrometer  ruled  in  squares  facilitated  the 
measuring  of  the  specimens.  The  following  species  have  been  des- 
cribed and  figured  ; Acroneuria  abnormis  and  a specimen  labeled 
as  this  species  purchased  from  an  American  dealer  but  which  has 
mandibles  of  a different  form  (Figs.  ),  Alloperla  minuta 

banks,  Capnia  necydaloides  Newn.,  Chloraperla  grammatica  Sep., 
Isopteryx  tripunctat  Sp.  Leuctra  Klapalekihny.  , wemura  varigata 
Oliv. , Perla  immarginata  nymph,  Perlesta  placida  nag.,  Pteronarcys 
regalis  Hewn. , nymph  and  adult,  and  Taeniopteryx  frigida  Hag. 

I wish  here  to  express  my  hearty  thanks  to  Professor  Alex. 

D.  MacGillivray , under  whose  direction  the  work  was  done,  for  his 
efforts  in  securing  material  for  study,  for  his  constant  interest, 
his  ever  ready  assistance  and  valuable  counsel  which  lightened  the 
routine  of  the  work,  and  also  for  the  permission  to  use  his  un- 
published terminology  much  of  which  appears  here  for  the  first  time, 
and  the  manuscript  of  his  forth  coming  book  on  the  external  mor- 
phology of  insects.  This  manuscript  proved  indispensable  as  a refer- 
ence and  of  intrinsic  value  as  a guide.  I am  exceedingly  grateful 


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to  Dr.  Hachiro  Yuasa  for  his  unstinted  kindness,  generosity  and 
skill  in  inking  my  drawings,  one  of  the  most  tedious  and  irksome 
tasks  connected  with  the  preparation  of  the  paper.  1 am  also  in- 
debted to  Professor  S.  A.  Forbes  for  the  use  of  specimens  from  the 
collections  of  the  Illinois  State  Laboratory  of  Natural  History, 
to  Dr.  C.  P.  Alexander  for  many  courtesies,  and  to  Mr.  Nathan 
Hanks  of  the M useum  of  Comparative  Zoology,  for  the  identification 
of  specimens,  i'he  European  species  studied  were  purchased  from  Dr. 
0.  Standinger  and  A.  Eang-Hass  of  Blasewitz  bei  Dresden,  Germany, 
ihese  specimens  were  identified  by  Dr.  Franz  Klapalek. 


FIXED  PARTS  OF  THE  HEAD, 


The  epicranial  suture  is  the  inverted  Y-shaped  line  on  the 
dorsal  aspect  of  the  head  usually  present  in  generalized  insects. 

It  marks  the  line  of  separation  of  the  sclerites  developed  from  the 
procephalon  from  the  sclerites  originating  from  the  cephalic 
lobes.  In  generalized  Exoptera  and  in  larvae  and  nymphs  the  suture 
is  usually  more  or  less  complete.  The  stem  is  the  first  portion  of: 
the  suture  to  disappear.  In  a hypothetical  type  of  a generalized 
insect  the  part  of  the  suture  which  is  called  the  stem,  begins 
as  a single  line  at  the  occipital  foramen  and  extends  cephalad 
to  a point  on  the  meson  between  the  compound  eyes,  where  it  branches 
dichotomously ; each  branch, which  is  known  as  an  arm,  extends 
laterad  and  cephalad  to  a point  on  the  f ronto- clypeal  suture  near 
the  precoila. 

In  the  Plecoptera  each  arm  of  the  epicranial  stem  always 
ends  at  a point  near  a lateral  ocellus  or  between  a lateral  ocellus 
and  the  mesal  or  cephalic  margin  of  a compound  eye.  The  nymph  of 
Pteronarcys  (Fig.  9 ) which  represents  the  most  generalized  type 
of  the  forms  studied,  has  a long  epicranial  3tem.  The  stem  in 
Acroneuria  (Fig.  3 ) is  shorter.  The  nymph  of  Perla  (Fig.  ) 
shows  a still  shorter  stem.  The  caudal  portion  of  the  stem  has  been 
retained  while  the  cephalic  portion  has  been  lost  in  Leuctra 
(Fig.  / ).  The  adult  Bieronarcys  (Fig.  fr'  ) shows  the  stem  almost 

obsolete.  In  Taeniopteryx  (Fig.  ^ ) the  stem  is  wanting  with  the 
exception  of  a short  cephalic  portion.  There  is  only  a slight 
indication  of  the  stem  in  Alloperla  (Fig.  y ).  The  cephalic  portion 
of  the  stem  is  obsolete  in  Chloraperla  (Fig. //  ) which  shows  a 


slight  indication  of  the  caudal  part.  The  stem  is  entirely  obsolete 

in  Capnia  (Fig.  /0  ) and  in  Nemjira  (Fig. ) Leuctra  (Fig.l)  and  Isop- 
teryx.  (Fig.  2). 

The  nymphs  of  Pteronarcys  (Fig.  ) and  Perla  show  the  long- 
est epicranial  arms  found  in  the  forms  studied.  Each  arm  extends 
to  the  cephalic  margin  of  a compound  eye  and  terminates  near  the 
antennaria.  The  arms  are  slightly  shorter  in  Acroneuria  (Fig.  3 ), 
each  ends  near  the  mesal  margin  of  a compound  eye.  Each  arm  of 
Leuctra  (Fig.  / ) extends  to  the  lateral  margin  of  a lateral  ocellus 

where  it  bends  abruptly  cephalad.  In  the  adult  of  Pteronarcys, 
Taeniopteryx,  Perlesta,  and  Capnia  (Figs.y,/^/oJeach  arm  ends  at  a 
point  near  a lateral  ocellus.  The  arms  of  Capnia  are  not  well 
developed.  In  Chlora^perla  (Fig.  //  ) the  mesal  portion  of  the  arms 
is  obsolete,  the  lateral  parts  are  distinct  altho  not  well  devel- 
oped. There  is  only  a slight  indication  of  the  arms  in  Alloperla 

(Fig.  7 ),  while  in  Isopteryx  (Fig.  ) and  in  Nemura  (Fig.  X ) 

<TT 

the  arms  are  entirely  obsolete.  The  vertex  is  one  of  the  paired 
sclerites  formed  from  the  cephalic  lobes.  In  the  generalized 
forms  of  the  Exoptera  the  epicranial  suture  is  present  and  separ- 
ates the  vertex  from  the  front  and  divides  the  sclerite  into  two 
parts.  The  vertex  extends  from  the  epicranial  arms  to  the  occip- 
ital suture.  It  bears  the  compound  eyes  and  extends  on  to  the 
ventral  aspect.  The  vertex  on  the  ventral  aspect  includes  the 
portion  of  the  head  between  the  antennaria  and  the  postgenae  and 
the  occiput.  The  comparative  size  and  shape  of  the  vertex,  when 
clearly  defined,  varies  only  slightly  in  the  Plecoptera  studied. 

The  vertex  in  the  nymph  of  Pteronarcys  (Fig.  ? ) is  larger 
than  in  most  of  the  other  forms  and  is  completely  divided  by  the 
epicranial  stem.  It  is  smaller  in  Acroneuria  and  in  the  nymph 


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of  Perla  (FigS.3, 6)  but  is  distinctly  divided  and  separated  from 
the  front  by  the  epicranial  suture.  The  two  halves  of  the  sclerite 
are  not  completely  divided,  due  to  the  partial  or  complete  absence 
of  the  epicranial  stem  in  Leuctra,  the  adult  of  Pteronarcys, 

Taeni  opteryx,  Capnia  and  Perlesta  (Figs./,  K /<£.)  . 

In  Chloraperla  (Fig.  t!  ) and  Alloperla  (Fig.  7 ) the  halves 
of  the  sclerite  are  not  completely  separated  due  to  the  partial 
or  total  absence  of  the  epicranial  stem.  The  sclerite  is  fused  to 
the  front  in  the  absence  of  parts  of  the  epicranial  arms.  There 
is  no  epicranial  suture  in  Isopteryx  (Fig.  S ) and  Nenfura  (Fig.  <2.  ) 
so  the  vertex  in  these  forms  is  not  divided  and  is  fused  with  the 
front.  In  Isopteryx  (Fig.  d* ) the  area  which  goes  to  form  the 
vertex  is  much  narrower  than  in  any  of  the  other  species.  The 
front  is  one  of  the  unpaired  sclerites  formed  from  the  procephalon. 
In  a hypothetical  type  of  a generalized  insect  and  in  some  of  the 
0 rthoptera  the  front  is  separated  from  the  vertex  by  the  epi- 
cranial arms,  from  the  genae  by  the  front o-genal  suture,  and  from 
the  clypeus  by  the  f ront o- clypeal  suture.  Some  of  the  Plecoptera 
studied  show  the  front  distinctly  separated  from  the  vertex  and 
from  the  clypeus,  in  other  more  specialized  forms  it  is  fused 
with  the  vertex  and  the  clypeus. 

The  nymph  of  Pteronarcys,  the  adults  of  Leuctra,  Taeni- 
opteryx,  Capnia,  and  Pteronarcys  (Figs.?  /,/y,/4#)  show  the  front 
distinctly  separated  from  the  vertex  and  from  the  clypeus.  The 
nymph  of  Perla,  Acroneuria,  and  Perlesta  (Figs. £.3,  /JL)  have  the 
front  separated  from  the  vertex  but  fused  with  the  clypeus.  In 
Nemura  (Fig.  X.  ) the  front  is  separated  from  the  clypeus  only. 
Chloraperla  (Fig.  II  ) and  Alloperla  (Fig.  7 ) have  the  front  al- 


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most  completely  fused  with  the  vertex  and  totally  fused  with  the 
clypeus.  The  front  apparently  occupies  the  greater  portion  of  the 
dorsal  aspect  of  the  head  in  Isopteryx  (Fig.  ) altho  it  is  com- 
pletely  fused  with  the  front,  the  genae,  and  the  clypeus. 

The  gena  is  not  a separate  sclerite.  The  term  is  applied 
to  the  lateral  area  of  the  head  cephalad  of  a compound  eye  and 
caudad  of  the  mandihle.  In  orthopterous  insects  the  genae  are  more 
or  less  completely  separated  from  the  front  by  the  fronto-genal 
sutures.  The  antennal  suture,  when  present,  separates  the  vertex 
from  the  antennaria.  The  Plecoptera  studied  show  this  suture  but 
the  genae  are  continuous  with  the  front  and  bear  the  antennariae. 

In  Acroneuria,  Chloraperla,  Perlesta,  Alloperla,  and  Isopteryx 
(Figs.3,//,/3, 6")  the  two  areas  are  easily  distinguished  by  a folding 
of  the  lateral  aspect  of  the  head  due  to  the  inturning  of  the  front 
and  the  clypeus  under  the  frontal  shelf. 

The  fronto-genal  sutures  are  the  lines  which  separate  the 
front  And  the  genae  in  the  generalized  Exoptera.  This  suture  is  the 
cephalic  portion  of  an  epicranial  arm  and  unites  with  the  fronto- 
clypeal  suture  at  a precoila.  Each  of  these  sutures  is  interrupted 
near  the  antennaria  and  the  lateral  ocellus  in  the  Orthoptera,  but 
the  cephalic  portion  of  the  suture  is  generally  very  distinct. 

Such  a suture  is  not  present  in  any  of  the  Plecoptera  examined.  The 
folds  between  the  genae  and  the  front  in  Acroneuria,  Perlesta, 

All opejr.la,  Choraperla  and  Isopteryx  (Figs.3/4,  >,//, should  not  be 
considered  as  fronto-genal  sutures  for  a smoothing  or  flattening 
of  the  lateral  aspect  of  the  head  removes  the  lines  made  by  the 
folding  of  this  region  and  reveals  the  surface  as  continuous  and 
uninterrupted  by  sutures. 


9 


The  compound  eye  is  a large  globular  projection  on  the  lat- 

V 

eral  aspect  of  the  head.  In  the  adults  examined  the  compound  eye 
occupies  a considerable  portion  of  the  lateral  aspect  of  the  dorsal 
and  the  ventral  surfaces.  In  the  nymphs  the  extent  of  the  compound 
eye  on  the  ventral  aspect  is  considerably  less  than  in  the  adult. 

The  oculata  is  usually  present  as  a narrow  band  surrounding 
the  compound  eye.  In  the  nymph  of  Perla  the  oculata  becomes  very 
much  wider  on  the  caudal  margin  of  the  eye  and  with  the  vertex  forms 
a sharp  ridge  which  is  fringed  with  numerous  stout  setae  on  the 
caudal  and  the  lateral  margin. 

The  three  ocelli  are  usually  located  on  the  front  or  the 
vertex  of  the  Exoptera.  The  median  ocellus  is  usally  situated  on 
the  front,  the  two  lateral,  on  the  vertex.  In  the  Plecoptera  exam- 
ined the  ocelli  are  located  on  the  front, the  two  lateral  generally 
in  close  proximity  to  the  epicranial  arms.  The  lateral  ocelli  in 
Isopteryx  (Fig.  if  ) are  large  and  much  nearer  the  occiforamen  than 
in  any  of  the  other  specimens  examined.  As  the  epicranial  arms  are 
obsolete  it  is  impossible  to  determine  the  caudal  extremity  of  the 
front.  The  length  of  the  vertex  has  been  greatly  reduced  or  the 
lateral  ocelli  have  migrated  caudad  on  to  the  vertex.  A study  of 
the  immature  forms  is  desirable  as  it  would  show  just  what  has 
occurred.  In  the  nymphs  (Figs. 6 , 9 ) studied  the  ocelli  are  very 

much  smaller  than  in  the  adult  forms  and  do  not  protrude  beyond 
the  surface  of  the  adjacent  areas.  On  the  adult  of  ? teronarcys, 

(Fig.  S'  ) as  in  other  forms,  the  ocelli  are  large,  globular  struc- 
tures which  project  as  distinct  convex  protuberances  beyond  the 
adjacent  surface.  The  margins  of  the  ocelli  in  the  nymphs  are  not 
as  sharply  defined  as  in  the  adults.  The  f&rm,  the  size,  the 


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absence  of  a definite  margin,  and  the  external  appearance  show 
clearly  that  the  ocelli  are  not  functional  in  the  nympha. 

The  antennaria  is  a small  sclerite  to  which  the  antacoria 
of  the  antenna  is  attached  in  generalized  insects.  Some  of  the 
largest  antennariae  are  found  in  generalized  Plecoptera  hut  in  some 
of  the  more  specialized  forms  of  this  order  the  antennariae  are 
very  rudimentary.  The  adult  of  Pteronarcys  has  the  largest  anten- 
nariae of  the  forms  studied.  The  nymph  of  P t eronarcy s (Fig.  ? ) 

and  Isopteryx  (Fig.  & ) have  well  developed  antennariae  which  are 
broadest  near  the  middle  on  the  dorsal  aspect  of  the  head.  Acron- 
euria  (Fig.  3 ) and  the  nymph  of  Perla  (Fig.  ) have  well  devel- 
oped antennariae  which  are  more  nearly  uniform  in  width  on  the 
dorsal  aspect.  Chloroperla  Taeniop  teryx,  and  beuctra  (Figs. //, 'V,  /) 
have  the  antennariae  more  reduced  at  the  caudal  extremity  and  lar- 
gest at  the  cephalic  end.  The  antennariae  of  Nemjira  (Fig.  3J  are 
uniform  in  width  on  the  dorsal  aspect  hut  are  distinctly  curved. 

The  arc  of  the  curve  is  directed  mesad.  Perlesta,  Alloperla,  and 
Capnia  ( Figs.  /3,  >,  /o)  have  antennariae  which  are  very  much  reduced. 
In  Capnia  they  are  quite  rudimentary  on  the  dorsal  and  the  ventral 
aspect,  while  in  Alloperla  and  Perlesta  they  are  somewhat  enlarged 
on  the  ventral  aspect  (Figs./tf,  /<2_  ). 

The  antacoria  is  the  membrane  between  the  antenna  and  the 
antennaria.  It  is  usually  quite  pronounced  on  the  caudal  and  the 
cephalic  aspects  and  very  narrow  between  the  antacoila  and  the 
antartis 

The  f ronto- clypeal  suture,  when  present,  separates  the  front 
from  the  clypeus.  The  primitive  PI  ecoptera  evidently  possessed  this 
suture  but  early  in  the  development  of  the  order  there  seems  to 


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have  been  a dichotomous  branching,  one  group  of  genera  retaining 
the  suture  and  developing  from  the  pretentoria  a lamella- like  ex- 
tension along  the  ental  surface  of  this  suture;  the  other  group 
of  genera  lost  the  suture  and  did  not  develop  the  lamella- like  ex- 
tension of  the  pretentoria.  The  genera  belonging  to  the  group  which 
lost  the  suture  are  described  first. 

ihe  nymph  of  Pteronarcys  (Fig.  9 ) has  a distinct  fronto- 
clypeal  suture.  On  the  ental  surface  of  the  head-capsule  there  is 
a slight  lamella,  a thickening  due  to  the  infolding  of  the  coria 
along  its  entire  length.  The  lamella  is  not  an  extension  of  the 
pretentoria.  Ihe  median  portion  of  the  suture  is  obsolete  in  the 
adult  (Fig.  9)  ) and  the  lateral  portions  are  only  faintly  indicated, 
i'he  lamella  has  entirely  disappeared.  The  nymph  of  Pteronarcys 
probably  resembles  the  condition  in  the  primitive  ancestors  before 
any  modification  had  taken  place.  The  disappearance  of  the  suture 
and  the  lamella  in  the  adult  seems  to  indicate  a beginning  of  the 
modification  which  has  been  continued  in  the  group  in  which  there 
is  no  suture.  Acroneuria,  the  nymph  of  Perla,  Chloraperla,  Isop- 
teryx,  Perlesta,  and  Alloperla  have  no  f r ont o- clypeal  suture.  The 
suture  and  the  tentorial  lamella  is  very  distinct  in  Leuctra.  Suc- 
cessive stages  in  the  enlargement  of  the  tentorial  lamella  along 
the  suture  are  seen  in  Capnia,  Taeni opteryx,  and  iveufura.  In  each 
of  these  forms  it  is  well  developed.  The  tentorial  lamella  is  a 
characteristic  feature  of  the  Orthoptera. 

The  clypeus  is  one  of  the  unpaired  sclerites  developed  from 
the  procephalon.  It  is  located  cephalad  of  the  front  and  caudad  of 
the  labrum.  In  the  more  generalized,  insects  it  is  separated  from 
the  front  and  from  the  labrum  by  distinct  sutures.  In  the  P 1 ecoptera 
there  is  no  indication  of  a subdivision  of  the  clypeus.  The 


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12 

clypeus  is  a large  well  defined  sclerite  in  the  nymph  of  Pteron- 
arcys  (Fig.  9 ).  It  is  separated  from  the  front  by  the  fronto- 

clypeal  suture  and  from  the  labrum  "by  the  clypeo-labral  suture. 

The  caudal  portion  is  covered  with  short  setae,  the  cephalic  por- 
tion is  glabrous.  The  sutures  separating  the  clypeus  from  the  front 
and  the  labrum  are  distinct  in  .Leuctra  and  Taeniopteryx  (Figs*/,  'V  ). 
In  Nemura  and  Capnia  (Figs.  <2.  , / 0 ) The  clypeus  is  much  broader  on 

the  caudal  margin  than  it  is  on  the  cephalic  and  is  bounded  by 
clearly  defined  sutures.  In  the  adult  of  Pteronarcys  (Fig.  Y ) the 
f ronto- clypeal  suture  is  almost  obsolete  and  the  clypeus  is  partly 
fused  with  the  front.  It  is  separated  from  the  labrum  by  a well 
defined  suture.  The  clypeus  is  fused  with  the  front  in  Acroneuria, 
the  nymph  of  Perla,  Chloraperla,  Isopteryx,  Perlesta,  and  Allo- 
perla  (Figs. 3, 6,  "'Sax.,  7). 

The  precoila  is  the  point  of  articulation  of  the  mandible 
on  the  dorsal  aspect  with  the  clypeus,  adjacent  to  the  lateral 
extremities  of  the  f ronto- clypeal  suture  when  this  suture  is  present 
In  orthopterous  insects  the  precoila  is  distinct  and  visible  from 
the  dorsal  or  cephalic  aspect.  In  the  Plecoptera  examined  the  pre- 
coila is  never  visible  from  the  dorsal  aspect.  The  nymph  and  adult 
of  Pteronarcys,  Leuctra,  Capnia  and  i\emura  ( Figs.?,?',  /,  (0 , ZL  ) have 
the  precoila  visible  only  from  the  lateral  aspect,  due  to  a bending 
of  each  lateral  portion  of  the  clypeus  and  sometimes  of  the  front 
ventrad.  It  is  at  this  ventral  edge  of  the  clypeus  that  the  precoila 
is  found  adjacent  to  the  f ronto- clypeal  suture,  when  it  is  present, 
which  also  bends  ventrad  with  the  clypeus.  In  the  nymph  of  Perla 
the  clypeus  bends  ventrad  and  mesad  and  it  is  under  this  ledge 
formed  by  the  clypeus  that  the  precoila  is  located,  ihe  front  and 
the  clypeus  bend  ventrad  and  mesad  forming  a similar  ledge  beneath 


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which  the  precoila  is  located  in  Acroneuria,  Isopteryx,  Alloperla, 
Chloraperla,  and  Perlesta.  The  f ront o- clypeal  suture  is  lacking  in 
the  forms  having  a frontal  ledge  as  has  "been  stated  previously. 

The  labrum  is  one  of  the  unpaired  sclerites  formed  from 
the  procephalon.  It  is  movable  and  is  the  upper  lip.  In  the  Plec- 
optera  it  is  separated  from  the  clypeus  by  a distinct  suture,  and 
in  the  forms  examined,  is  a well  defined  and  usually  a large  area. 
The  labrum  is  a distinct  quadrangular  sclerite  in  the  nymph  of 
Pteronarcys.  Its  caudal  portion  is  densely  setaceous,  the  cephalic 
portion  is  bare.  In  Taeni  apteryx,  Isopteryx,  Alloperla,  and  Chlora- 
perla the  labrum  is  of  the  same  general  shape  as  in  the  nymph  of 
Pteronarcys  ( Pigs. 7^^  7,  //  )•  The  posterior  portion  is  more  or 

less  membranous.  The  labrum  of  heuctra  (Fig.  ( ) and  Capnia  (Fig.  JO) 
has  the  same  general  shape  as  in  the  preceeding  group,  but,  due 
to  a bending  ventrad,  appears  short  from  the  dorsal  aspect.  The 
labrum  in  the  adult  of  Pteronarcys  (Fig.  ) and  Perlesta  (Fig.  /-2. ) 
is  crescentic  and  narrow,  when  measured  on  the  meson:  the  labrum 
bends  ventrad  and  from  the  dorsal  aspect  appears  narrower  and  more 
crescentic  than  it  really  is.  This  is  strikingly  true  of  h'enmra  in 
which  the  labrum  is  almost  as  long  as  broad  (Fig.  ,2.60  but  due  to 
a ventral  incurving  appears  very  narrow  from  the  dorsal  aspect 
' Fig.  CL  ) . In  the  nymph  of  Perla  (Fig.  i?  ) the  labrum  is  biemar- 
ginate  on  the  cephalic  margin  and  the  mesal  lobe  bears  a small 

membranous  flap  which  is  densely  setaceous.  The  labrum  in  Acron- 
euria is  very  much  smaller  than  in  any  of  the  other  species 
examined. 

ihe  clyp eo- labral  suture  is  the  line  or  coria  between  the 
clypeus  and  the  labrum.  It  is  present  in  all  of  the  forms  studied. 


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In  the  nymph  of  Perla  (Fig.  ^ ) it  is  long  and  narrow;  while  in 
the  nymph  of  Pteronarcys  (Fig.  2 ) it  is  wider  and  membranous. 

It  is  membranous  in  Acroneuria  (Fig.  Q ) and  contains  a small 
slightly  crescent- shaped  secondary  sclerite  or  thickening  near  the 
cephalic  extremity  in  close  proximity  to  the  labrum. 

The  tormae  are  thickenings  between  the  clypeus  and  the  lab- 
rum  at  the  lateral  extremities  of  the  clypeo-labral  suture.  They 
are  well  developed  in  all  of  the  specimens  examined  with  the  single 
exception  of  Acroneuria.  Here  they  are  rudimentary  and  consist  of 

an  unbranched  thickening  which  extends  caudad  from  the  labrum.  In 

and  are 

all  of  the  other  forms  studied  the  tormae  are  conspicuous/usually 
branched  (Fig.^^A^  $ Zj. 

The  opening  in  the  caudal  aspect  of  the  head  thru  which  the 

nerve 

alimentary  canal-  cord,  and  other  organs  pass  into  the  cervix  is 
the  occipital  foramen.  In  the  cockroach  the  occipital  foramen  is 
located  on  the  mesal  and  caudal  portion  of  the  ventral  aspect  of 
the  head.  The  vertex  and  the  occiput  extend  on  to  the  ventral  aspect 
caudad  of  the  occipital  foramen  and  in  no  instance  is  the  occipital 
foramen  visible  from  the  dorsal  aspect.  In  the  Plecoptera  studied 
the  occipital  foramen  is  located  on  the  caudal  portion  of  the  ven- 
tral aspect  and  occupies  the  mesal  portion  of  the  caudal  aspect  of 
the  head  extending  caudad  and  dorsal,  the  caudal  portion  always 
being  visible  from  the  dorsal  aspect.  In  the  more  generalized  Stone- 
flies  a large  part  of  it  is  on  the  ventral  aspect.  In  the  special- 
ized genera  the  portion  on  the  ventral  aspect  is  smaller  and  the 
caudal  portion  extends  further  on  to  the  dorsal  aspect.  The  posi- 
tion of  the  occipital  foramen  in  the  Plecoptera  is  intermediate 
between  the  forms,  in  which  the  occipital  foramen  is  located  on  the 


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ventral  aspect  as  in  the  Orthoptera  and  not  opposite  the  mouth  open- 
ing, and  those  in  which  the  occipital  foramen  is  limited  to  the 
caudal  aspect  and  opposite  the  mouth  opening. 

The  most  generalized  condition  of  the  occipital  foramen  in  the 
Plecoptera  is  found  in  Leuctra  (Fig.  3.0" ).  The  occipital  foramen  is 
ss  long  as  broad  and  roughly  quadrangular  in  shape.  The  length  of 
the  occipital  foramen  in  Taeniopteryx  (Fig.  3 3.  ) is  greater  than 
the  breadth.  Specialization  is  shown  in  the  migration  of  the  occ- 
ipital foramen  caudad  and  dorsal.  The  nymph  of  Pteronarcys,  Capnia, 
the  adult  of  Pteronarcys,  Isopteryx,  Alloperla,  Nemura,  chloraperla, 

A 

Acroneuria,  Perla,  and  Perlesta  ( Figs. 2?,  3d,  Q.°ll  3^, 36,2.6)  show  increas- 
ing specialization.  Perlesta  (Fig.  ) has  the  most  specialized 
occipital  foramen  of  the  forms  figured.  Here  the  length  is  much  less 
than  the  breadth.  On  the  dorsal  aspect  the  vertex  and  the  occiput 
are  distinctly  emarginate. 

The  odontoidea  is  the  point  of  articulation  of  a cerv  e-pister- 
num  with  the  head-capsule.  Each  is  a triangular  projection  located 
on  the  lateral  edge  of  the  occipital  foramen.  In  the  Plecoptera  the 
odontoidea  is  usually  well  developed  and  distinct. 

The  occipital  suture  is  not  present  in  the  nymphs  of  Pteronar- 
cys and  Perla,  and  in  Leuctra  and  Nemura  (Figs.aX,  dfj  . In 

Capnia  (Figf  36}  the  suture  is  obsolete  on  the  dorsal  aspect  but 
is  clearly  defined  on  the  ventral  aspect  opposite  the  mesal  portion 
of  a compound  eye.  In  Pteronarcys,  Taeniopteryx,  Acroneuria,  Isop- 
teryx, Chloraperla,  Alloperla,  and  Perlesta  ( Figs.<*%33( i'>,  35) 

the  occipital  suture  is  distinct  on  the  dorsal  aspect  but  is  indis- 
tinct opposite  a metatent orina  and  is  difficult  to  identify  on 
account  of  the  folding  of  the  cuticle  on  this  area.  In  most  of  the 


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16 

last  mentioned  forms  the  occipital  suture  can  he  identified  thruout 
its  course.  (i*'igs.  27,2.  ^.3  CT , -3  &) • 

The  postgena  is  the  area  on  the  ventral  aspect  of  the  head 
mesad  of  the  occipital  suture,  when  this  suture  is  present,  extend- 
ing to  the  postcoila  and  fusing  with  the  occiput  near  a metatento- 
rina.  In  Capnia  (Fig.  SO  ) there  is  an  indication  of  a line  which 
extends  from  the  cephalic  margin  of  a met atent orina  to  the  occipi- 
tal suture.  This  line  is  undoubtedly  secondary  and  is  not  found  in 
any  of  the  other  forms  examined.  Its  position  near  a metatent orina 
also  suggests  that  it  is  secondary.  In  the  nymphs  of  Pteronarcys 
and  Perla,  and  in  peuctra  and  hemura  (Figs.?S^3  1,2^/  ^6)  the  post- 
genae  are  not  separated  from  the  vertex  or  the  occiput.  Each  post- 
gena is  more  or  less  completely  separated  from  the  vertex  hy  sin 
occipital  suture  in  Taeni opteryx,  Acroneuria,  Isopteryx,  Chlora- 
perla,  the  adult  of  Pteronarcys,  Perlesta,  and  Alloperla  (Figs. 0-2., 
2-7,  3^,  3 3,  Z.°I,  id,  367. 

The  occiput  is  one  of  the  paired  sclerites  formed  from  the 
cephalic  lobes.  It  is  the  area  between  the  vertex  and  the  occipital 
foramen.  The  suture  separating  the  vertex  from  the  occiput  is  often 
wanting.  In  the  nymphs  of  Pteronarcys  and  Perla,  and  in  Leuctra  and 
iiemura  (Figs.  If,  3 the  occiput  is  indist  inguishably  fused  with 

the  vertex.  In  Capnia  the  occiput  is  also  fused  with  the  vertex. 

The  occiput  and  postgenae  are  more  or  less  completely  separated 
from  the  vertex  by  the  occipital  suture  in  Taeniopteryx,  Acroneuria, 
Isopteryx,  chloraperla,  Alloperla,  Perlesta,  and  Pteronarcys 
( jfigs.93,  A}, ‘34^33 36)  • On  the  dorsal  aspect  the  occiput  is  a very 
narrow  band  which  becomes  enlarged  on  the  lateral  aspects  adjacent 
to  the  lateral  portions  of  the  occipital  foramen. 


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17 


The  crassa  is  present  in  all  of  the  forms  studied.  It  usually 
extends  from  near  the  central  portion  of  a postcoila  to  a point 
opposite  the  cephalic  margin  of  a compound  eye. 

The  paracoila  is  the  point  of  articulation  of  a maxilla  on 
the  ventral  aspect.  In  the  Plecoptera  it  is  always  at  the  cephalic 
margin  of  a metatent orina.  The  maxillaria  is  not  well  developed  in 
the  Plecoptera. 

The  postcoila  is  the  point  of  articulation  of  a mandible  on 
the  ventral  aspect  at  the  cephalic  margin  of  a postgena.  as  a rule 
the  relative  distance  between  the  postcoila  and  the  paracoila  is 
much  less  in  generalized  than  in  specialized  forms.  In  the  Plecop- 
tera this  is  illustrated  by  referring  to  figures«itf*  and3o  .Leuctra 
(Fig.  ) is  a generalized  form  in  which  the  distance  between  the 
postcoila  and  the  paracoila  is  just  one-half  that  between  these 
points  in  Perlesta  (Fig. 3 0 ) which  represents  a specialized  con- 
dition. A comparison  of  figures  shows  that  the  other  species  studied 
are  intermediate  between  these  forms.  In  a linear  series  the  speci- 
mens would  be  arranged:  Leuctra,  Pteronarcys  (nymph),  jNem^ura, 
isopteryx,  Acroneuria,  Pteronarcys  (adult),  Perla,  Taeniopteryx, 
Capnia,  Alloperla,  Chloraperla,  Perlesta. 

The  tentorium  is  the  end oskel eton  of  the  head  and  is  formed 
from  paired  invaginations  of  the  head-capsule.  These  invaginations 
of  the  wall  of  the  head  extend  entad,  meet,  and  fuse,  forming  a 
strong  framework  for  the  attachment  of  muscles  and  tendons  and  for 
the  support  of  the  internal  organs  of  the  head.  The  fused  portion 
of  the  tentorium  is  usually  called  the  body  and  the  three  pairs  of 
projections  extending  from  the  head-capsule  to  the  fused  portion  are 
called  the  arms. 


f 

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One  pair  of  invaginations,  the  metaten torina* , occurs  on  the 
ventral  aspect  where  they  are  considered  to  have  originated  as  apo- 
demes  of  the  maxillary  segment.  The  cephalic  pair  of  invaginations, 
the  pretentorinae  occurs  on  the  dorsal  or  later o-dor sal  aspect 
near  the  cephalic  extremities  of  the  epicranial  arms  and  the  lateral 
limits  of  the  f ronto-clypeal  suture,  when  this  is  present.  Each 
pretentorina  is  usually  also  closely  associated  with  a precoila. 
Carriere  and  burger  concluded  from  their  embryologi cal  studies  that 
each  pretentorium  originates  from  a spiracle  of  the  mandibular  seg- 
ment while  Riley  is  of  the  opinion  that  the  originate  as  apodemes. 
The  third  pair  of  arms,  the  supratentoria,  are  attached  to  the  head- 
capsule  on  the  dorsal  aspect  cephalad  and  laterad  of  a lateral 
ocellus  and  fuse  with  the  pretentoria  cephalad  of  the  metatentoria 
in  the  Plecoptera.  The  supratentoria  are  said  by  some  students  to 
be  projections  of  the  pretentoria,  others  suggest  that  they  origin- 
ate as  invaginations  similar  to  those  of  the  posterior  and  anterior 
arms. 

Erom  embry ological  studies  of  Blatta  it  has  been  pointed  out 
that  the  invaginations  for  the  posterior  pair  of  arms  are  much  less 
prominent  than  those  of  the  anterior  arms,  and,  that,  due  to  an 
apparent  retardation  in  their  development  in  embryos  which  clearly 
showed  the  invaginations  for  the  anterior  arms  in  the  mandibular 
segment,  the  invaginations  for  the  posterior  arms  could  not  be  iden- 
tified. The  pretentorinae  are  small  in  the  embryo  of  Blatta  altho 
the  arms  are  well  developed  in  the  adult.  The  metatentoria  and  the 
metatentorinae  are  also  quite  distinct  and  large  in  the  adult,  tho 
as  has  been  pointed  out  by  Riley,  it  is  sometimes  impossible  to 
identify  them  on  embryos  that  show  the  pretentorinae.  As  the 


19 


supratent oria  are  very  much  reduced  in  the  adult  of  filatta  the 
aupratentor ia  and  the  supratent orinae  must  he  much  smaller  in  the 
embryo  and  probably  somewhat  more  retarded  in  their  development 
than  the  posterior  arms,  and  therefore,  easily  overlooked.  The 
supratentoria  in  Blatta  are  very  much  smaller  than  in  Melanoplus, 
Gryllus,  or  Diapheromera  which  have  the  supratentoria  firmly  att- 
ached to  the  head-capBule  and  larger  at  the  point  of  attachment 
than  near  the  pretentoria.  In  the  Plecoptera  the  adult  of  Pteron- 
arcys  (Pig.  y.  ^ ) and  Isopteryx  (Pig.  3V  ) are  the  only  forms  exam- 

ined in  which  the  supratentoria  are  smaller  at  the  head- capsule 
than  toward  the  body  of  the  tentorium  as  described  by  Comstock  and 
Kochi,  while  in  Leuctra  (Pig.  2.  C ),  which  appears  to  be  one  of  the 
most  generalized  forms  examined,  the  supratentoria  are  greatly 
enlarged  at  the  head-capsule. 

As  all  studies  of  the  morphology  of  the  cockroach  have  been 
based  on  three  or  four  closely  related  domestic  species,  is  it  not 
possible  that  more  generalized  species  have  large  supratentoria  like 
Melanoplus,  or  Gryllus  or  even  like  some  of  the  generalized  Plecop- 
tera and  may  they  not  be  found  on  further  study  of  other  forms  to 
be  invaginated?  The  subject  offers,  at  least,  a broad  and  rich 
field  for  further  investigation. 

In  many  insects  the  metatent orinae  appear  as  open  pockets  be- 
tween the  postgenae  and  the  maxillariae.  In  the  Plecoptera  the 
metatent orinae  are  very  prominent  and  distinct.  They  are  largest  in 
Acroneuria  (Pig.  S'?  ) and  Chloraperla,  (Pig.  3 3 ) and  most  reduced 

in  Taeniopteryx  (Pig.  3 2.)  and  Nemura  (Pig.  *16  ).  The  unfused  por- 

A. 

tion  of  this  invagination  within  the  cavity  of  the  head,  on  each 
side,  is  a metatentorium;  the  fused  portion  is  the  corpotent orium. 


20 


In  the  Plecoptera  examined  the  metatentoria  are  seldom  very  con- 
spicuous. In  the  nymphs  of  Perla  and  Pteronarcys,  and  in  Taeniop- 
teryx,  Nem^.ra,  Leuctra,  Isopteryx,  and  Capnia  (Figs.3  ijsl  '3  2/:L6fL6 
the  metatentoria  extend  around  the  occipital  foramen  forming  a 
slightly  thickened  ridge  on  the  ental  surface. 

The  cephalic  arms  of  the  tentorium  are  the  preten toriae,  and 
originate  as  invaginations  which, in  generalized  insects  are  usually 
located  on  the  dorsal  aspect  of  the  head.  In  the  Plecoptera  exam- 
ined each  pretent orina  iB  located  at  the  lateral  extremity  of  the 
f ronto- clypeal  suture,  when  it  is  present,  hut  on  the  lateral  as- 
pect or  beneath  the  frontal  ledge  which  is  formed  by  a bending  of 
the  front  and  clypeus  ventrad  and  mesad.  In  none  of  the  specimens 
examined  are  the  pretent orinae  visible  from  the  dorsal  aspect.  In 
Leuctra,  hemura,  Capnia,  the  adult  of  Pteronarcys,  Isopteryx,  and 
Taeniopteryx  (Figs.*^^,36/-2-?  3^32.)  each  pretentorium  is  short,  very 
much  flattened,  and  broadly  triangular  in  outline  at  the  peripheral 
end.  One  angle  of  the  plate  is  adjacent  to  the  precoila,  one  ex- 
tends laterad  and  dorsal  toward  an  antafossa,  and  the  third  to  the 
laminatent orium.  Each  pretentcrium  is  longer  and  slenderer  in  the 
nymph  of  Pteronarcys,  Chloraperla,  and  Alloperla  (Figs. AS,  33,  36  ) 
than  in  the  preceeding  group  but  they  have  the  same  general  form  as 
found  in  that  group.  Acroneuria,  the  nymph  of  Perla,  and  Perlesta 
(Figs.  ^ \ b)  have  long,  slender,  rod-like  pretentoria. 

Each  supratent orium  is  attached  to  the  head- capsule  on  the  dor- 
sal aspect  cephalad  and  laterad  of  a lateral  ocellus  and  fuses  with 
a pretentorium  cephalad  of  the  metatentorium  in  the  Plecoptera. 
Leuctra  (Fig.  ) which  appears  to  be  one  of  the  most  generalized 
forms  studied,  and  Capnia  (Fig.  3 o ) show  the  supratent oria, 


, 


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. 


greatly  enlarged  at  the  head- capsule.  Acroneuria  (Fig.  a ) and  the 
nymph  of  Perla  (Fig.  ^ / ) have  cluh-shaped  suprat ent oria  with  the 
large  part  of  the  club  toward  the  head- capsule.  The  suprat entori a 
of  Taeni  opteryx,  Nem^ira,  Chloraperla,  Alloperla,  and  Perlesta, 

(Figs . 3 ^6.53^4,**)  are  approximately  uniform  in  size  thruout  their 

length,  while  Isopteryx,  (Fig.^^  ) and  the  nymph  of  Pteronarcys 
(Figs,  j ^ ) have  the  supratent oria  much  smaller  at  the  head- 

capsule  than  at  the  body  of  the  tentorium. 

The  laminatent orium  is  formed  by  a fusion  of  the  pretentoria 
cephalad  of  and  adjacent  to  the  corpotentorium.  This  fusion  is 
usually  indicated  by  a line  or  ridge  on  the  meson.  The  laminatent or- 
ium is  long  and  the  line  of  fusion  is  distinctly  indicated  on  the 
meson  in  Taeni opteryx,  Leuctra,  Capnia,  Chloraperla,  and  Alloperla 
(Figs.3^,*F  . In  Isopteryx  (Fig.^  ) it  is  narrow.  The  lam- 

inat ent orium  is  longer  than  the  corpotentorium  in  the  nymph  of 
Perla  (Fig.  3/  ).  The  laminatent or ium  and  the  corpotentorium  are 
indistinguisedably  fused  in  the  nymph  of  the  adult  of  Pteronarcys, 
hemjira,  Acroneuria,  and  Perlesta  ( Fig/^^6,  * >,  53”)  , 

The  corpotentorium,  which  is  formed  by  the  fusion  of  the  mesal 
portions  of  the  metatentoria  and  which  fuse  with  the  laminatent orium 
never  has  any  indication  of  a line  of  fusion  on  the  meson.  In  the 
Plecoptera  it  can  usually  be  distinguished  from  the  laminatent orium, 
measured  on  the  meson,  is  short  in  Leuctra,  the  nymph  of  Perla, 

Taeni  op  teryx,  Capnia,  Alloperla,  and  Chloraperla,  (Figs.*-  36}  3 j) 

is  larger  in  Isopteryx  (Fig.  ZH  ).  There  is  no  indication  of  a 
line  of  fusion  of  the  corpotentorium  and  the  laminatent orium  in  the 
nymph  and  adult  of  Pteronarcys,  .wemjira,  Acroneuria  and  Perlesta 
(Figs.ijja^u^.  3 &). 


. 

- 

. 

. 

. 

• 

, 

. 


. 


. 

. 


. 

. 


. 


. 


. 


'X;"  . 


, 

. 


. 


. 


! 

4 * 


. 


22 

The  laminatent orium  hears  a single  distinct  cesctendcn  on  the 
cephalic  margin  in  the  nymph  of  Perla,  Perlesta,  Chloraperla,^ty. 

x 'j) 

and  Acroneuria;  in  Isopteryx  and  Alloperla  (Figs. 3^  36)  it  is  rudi- 
mentary. In  the  nymph  of  Perla  and  Acroneuria  (Figs.  3 } A > ) 

there  are  two  flexotendons  on  the  ventral  surface  of  the  corpoten- 
torium  near  the  laminatent crium  which  extend  ventrad. 


- 


. 


MOVABLE  PARTS  OP  HEAD 


23 

The  mandibles  are  appendages  of  the  fourth  segment  of  the  head, 
xhe  form  of  the  mandibles  varies  in  the  different  orders  and  in 
the  species  of  the  same  order  according  to  the  food  habits  of  the 
insects.  A reduction  in  the  size,  cutting  edge,  and  strength  indic- 
ates specialization  in  food  habits. 

In  the  mandibles  the  nymphs  figured  are  typically  orthopteran 
in  form,  Those  of  the  nymph  of  Pteronarcys  (FigA  are  slightly 

broader  than  long  with  the  lateral  edge  of  the  proximal  end,  the 
distance  between  the  preartis  and  the  postartis,  equal  to  at  least 
two- thirds  of  the  entire  length  of  the  mandible  while  the  width 
of  the  distal  end  is  slightly  less  than  one-half  that  of  the  prox- 
imal end.  The  dorsal  aspect  is  convex,  the  ventral  is  slightly 
concave.  The  dentes  or  teeth  are  well  developed  and  heavily  chit- 
inized.  The  smooth  rounded  protuberance,  the  mola, is  present  at 
the  proximal  end  of  the  dentes  on  the  ventral  aspect.  The  setae 
near  the  mola,  the  inf ra^brust iae , are  prominent,  rhe  rectotendon 
is  large  and  is  attached  to  the  mesal  edge  of  the  proximal  end. 

The  extensotendon  is  slender  and  is  attached  on  the  dorsal  aspect  iro 
the  latero-ventral  extremity.  The  point  of  articulation  of  a man- 
dible on  the  dorsal  aspect  with  a precoila  is  a preartis.  It 
is  adjacent  to  the  extensotendon  and  is  convex,  in  striking  contrast 
to  the  globular,  protruding  postartis,  the  point  of  articulation 
with  a postcoila  on  the  ventral  aspect.  The  mandibles  of  the  nymph 
of  Perla  (Fi gS.6Ln<*)  show  the  same  general  condition  as  described 
above.  The  length  exceeds  the  breadth  and  the  mandible  is  thinner 
and  more  convexo-concave  than  in  the  nymph  of  Pteronarcys.  The  mola 
is  not  present. 

rhe  form  of  the  mandibles  of  the  adults  varies  greatly..  This 


r -» 


, 


, 


. 


, 


. 


. 


. 

. 


. 


. 


. 


variation  is  strikingly  shown  in  the  features  of  the  lateral  as- 
pects. Kem^ira,  Capnia,  Leuctra,  and  Isopteryx  (Figs.  6<>) 

have  mandibles  very  similar  to  those  of  the  nymph  of  Pt eronarcys. 
in  Nemura  (Fig.  ^2-)  the  distance  between  the  preartis  and  the 
postartis  is  as  shown  on  the  lateral  aspect,  equal  to  the  width  of 
the  mandible,  while  in  Alloperla  (Fig.  (s>°l  ) the  distance  between 

the  preartis  and  the  postartis  exceeds  the  width  of  the  proximal 
end.  fhe  distal  end  is  greatly  reduced  but  the  dentes  are  sharp  and 
prominent.  The  mandible  of  the  adult  of  Pteronarcys  (Fig.  ) has 
no  prominent  angularities,  is  broader  than  long  and  rudimentary  in 
appearance.  A very  different  type  is  found  in  Acroneuria,  Perlesta, 
and  Chloraperla  (Figs.  6%,  ) • In  these  species  the  mandible  is 

not  heavily  chitinized  and  are  longer  than  wide.  The  distance 
between  the  preartis  and  the  postartis  varies  but  the  proximal  end 
is  always  much  wider  than  the  distal  which  is  always  thin  and  bears 
from  one  to  several  dentes. 

The  most  generalized  condition  of  the  mandibles  of  any  of  the 
species  figured  is  found  in  the  nymph  of  Pteronarcys  (Fig.  \y  ). 
They  are  typically  orthopteran  in  form.  The  lateral  aspect  is  very 
broad  at  the  proximal  end  and  is  one-half  as  wide  at  the  distal 
end.  The  same  general  condition  is  found  in  Nemura,  Taeniopteryx 
and  Capnia,  (Figs.4:^,  V ) . Leuctra  and  Isopteryx  (Figs.^?.  &&  ) 

have  mandibles  of  the  same  general  type  as  those  described  above 
but  the  distal  end  is  narrower,  tho  not  printed,  and  the  mandibles 
are  more  curved.  The  mandibles  of  the  adult  of  Pteronarcys  and  the 
nymph  of  Perla  (Figs.  ; c'Jr'  ) taper  to  a distinct  point  as  seen 
from  the  lateral  aspect.  Perla  is  distinctly  convexo-concave.  The 
mandibles  described  above  are  well  chitinized.  Two  specimens 


. 


■ 

- 


. 


. 


> I 


, 

. 

• 

- 

, 

, 


*-!? 

labeled  Acroneuria  abnormis  ( Figs.  ) are  shown  as  the 

mandibles  are  very  different  in  shape.  They  are  only  slightly  chit- 
inized,  are  broad  at  the  proximal  end  and  from  the  lateral  or  mesal 
aspect  taper  to  a sharp  point  at  the  distal  end.  The  mandibles  of 
Alloperla  (Fig.  b°t  ) are  distinctive  in  having  the  proximal  end 
wider  on  the  lateral  than  on  the  dorsal  ventral  aspect.  They  do  not 
taper  to  a distinct  point  as  in  Acroneuria,  Chloraperla  and  Per- 
lesta  but  agree  with  these  forms  in  being  only  slightly  chitinized. 
chloraperla  and  Perlesta  (Figs.  72.  , ) are  very  broad  at  the 

proximal  end  but  are  much  narrower  at  the  distal  end  of  the  prox- 
imal half.  The  distal  half  is  dagger-like  in  form.  These  two  species 
have  the  most  specialized  mandibles  figured. 

The  maxillae  are  appendages  of  the  fifth  segment  of  the 
head.  In  the  Plecoptera  the  maxillae  are  typically  orthopteran 
in  form.  The  generalized  species  have  all  the  parts  of  a normal 
generalized  type,  but  the  specialized  species  show  a considerable 
degree  of  fusion  of  the  different  parts,  and  many  sutures  which  in 
generalized  species  are  prominent  are  entirely  wanting  in  the  more 
specialized  forms.  A subcardo,  alacardo , stipes,  subgalea,  pal- 
pifer,  labial  palpus,  lacinia,  proxagalea,  and  distagalea  are 
present  in  most  generalised  forms. 

Leuctra  (Figs.  76,  77)  has  a generalized  maxillae,  ana 
shows  all  the  different  parts.  The  subcardo,  which  bears  the 
parartis,  is  large  and  distinctly  separated  from  the  alacardo  which 
is  also  large  and  prominent.  The  parartis  is  the  chitinized  area 
on  the  dor  s al  a sp  e c t of  the  subcardo  which  articulates  with  a 
paracoila.  The  exparartis  (Fig.  86)  is  at  the  proximal  end,  and 
is  enlarge^,  slightly  concave,  and  articulates  with  the  outside  of 


26 

the  head-capsule  at  the  paracoila.  The  entopararti a is  tne  end 
which  bears  the  premaxs©:endon  and  articulates  on  the  ental  sur- 
face. The  premaxa tendon  is  long  and  slender.  It  is  attached  to 
the  entoparartis  in  close  proximity  to  the  alacardo  and  the  rnaxa- 
coria.  The  parts  of  the  parartis  are  not  well  developed. 

The  stipes  extends  onto  the  dorsal  aspect  but  is  much 
narrower  than  on  the  ventral  aspect.  It  bears  long  setae  on  the 
lateral  aspect.  The  subgalea  is  the  long  triangular  sclerite  on  tae 
ventral  aspect  between  the  stipes  and  the  labicoria.  The  palpifer 
is  not  prominent.  It  bears  the  palpus  which  is  twice  as  long  as 
the  maxilla.  The  proxagalea  is  distinctly  separated  from  tne 
distagalea  and  is  not  heavily  chitinized  on  the  ventral  aspect. 

The  distagalea  is  sickle-shaped  and  is  setiferous  on  the  dorsal  and 
lateral  aspects.  It  bears  a row  of  seta  on  the  ventral  surface  of 
the  mesal  edge  which  is  grooved.  The  lacinia  is  thin  and  shorter 
than  the  galea,  the  mesal  edge  is  sharp,  and  a lacinara.stra  is 
present.  The  lacinia,  fits  into  the  groove  of  the  galea.  The  maxa- 
coris.  and  the  labiacoria  are  distinct. 

are  distinct 

The  subcardo  and  the  alacardo. /in  the  nymph  of  Perla  (Pig. 
79),  Isopteryx  (Pig.  8l),  Taeniopteryx  (Pig.  83),  kemura  (Fig.  85), 
Capnia(Fig.  88),  and  Pteronarcus,  nymph  (Fig.  91),  and  adult  (Fig. 
90).  In  Chloraperla  (Fig.  95)  the  subcarda  is  present  but  not  as 
distinctly  separated  from  the  alaca.rdo  as  in  the  preceding  species. 
In  Alloperla  (Fig.  97),  Perlesta  (Fig.  100)  , and  Acroneuria  (Figs. 
93,  9 4),  the  subfardo  is  completely  fused  with  the  alacardo,  and  in 
the  latter  there  is  only  a slight  indication  of  a separation  of  the 
cardo  as  a whole  from  the  stipes. 

The  stipes  is  large  and  densely  setiferous  in  most  of  the 


n. 


27 


species.  In  the  nymph  of  Perla  (Pig.  78)  its  mesal  extremity  on  the 
dorsal  aspect  bears  a conspicuous  row  of  stout  setae  which  extends 
from  the  alacardo  to  the  palpifer.  The  stipes  is  partly  fused 
with  the  subgalea  on  the  ventral  aspect  in  the  nymph  cf  Perla  (Pig. 
79) » Acroneuria  (Pig.  94),  and  Chloraperla  (Pig.  95).'  In  Isop- 
teryx (Pig.  8l ) , Nemura  (Pig.  85),  and  Perlesta  (Pig.  100),  it  is 
completely  fused  with  the  subgalea.  The  stipes  articulates  with 
the  alacardo  on  the  dorsal  aspect  and  with  the  subcardo  on  the  ven- 
tral aspect.  The  palpifer  is  distinct  in  most  of  the  species  fig- 
ured. It  is  trie  rounded  protuberance  which  bears  the  palpus. 

The  galea  is  divided  into  two  segments  in  the  generalized 
forms.  In  Leuctra  (Pigs.  76,  77),  Isopteryx  (Pigs.  80  , 8l),  Taeniop- 
teryx  (Pigs.  82,  83),  Nemura  (Fjfcgs.  84,  85),  Capnia  (Pigs.  87,  88), 
and  Pteronarcys  nymph  (Pig.  91) , and  adult  (Pig.  90) , the  proxagalea 
'is  shorter  than  the  distagalea.  In  the  nymph  of  Perla  (Pig.  78) 
the  distagalea  is  slender  and  shorter  than  the  proxagalea  and  re- 
sembles in  general  form  the  galea  of  Harpalus,  a carabid.  The 
distagalea  is  enlarged  at  the  distal  end  in  Isopteryx  (Fig.  8l). 
There  is  a dense  cluster  of  setae  near  the  end  of  the  distagalea  in 
Pteronarcys  numph  (Pig.  91)  2-nd  adult  (Fig,  90).  The  proxagalea 
and  the  distagalea  are  indistinguishably  fused  in  Acroneuria  (Pig. 
93),  Chloraperla  (Pig.  96),  Alloperla  (Pig.  98),  and  Perlesta  (Pig. 
99). 

The  lacinia  is  thin  and  the  mesal  edge  is  sharp.  The 
setae  of  the  lacinarastra  are  stout  in  Isopteryx  (Pig.  80)  and 
ITeirftira  (Pig.  84),  long  in  the  numpt  cf  Perla  (Pig.  78),  slightly 
shorter  in  the  nymph  of  Pteronarcys  (Fig.  92)  but  promiscuously  ar- 


' 

' 


. 

. 

‘ 


28 

ranged.  The  lacinarastra  is  present  in  Taeniopteryx  (Fig.  82) , 
Capnia  (Fig.  87),  the  adult  of  Pteronarcys'  (Fig.  90)  , Chloraperla 
(Fig.  96)  and  Acroneuria  (Fig.  94-)  » in/the  last  two  it  is  greatly  re- 
duced. Alloperla  (Fig.  98)  and  Perlesta  (Fig.  99)  have  ncjlacinar- 
astra.  Capnia  (Fig.  88)  and  thd  nymph  of  Pteronarcys  (Fig.  92) 
have,,  distinct  maxadentes.  There  is  a distinct  movable  hook  between 
the  lacinaristra  and  the  distal  end  of  the  lacinia  which  extends  as 
a sharp  curved  projection  beyond  the  end  of  the  hook.  The  lacinia 
is  longer  than  the  galea  in  the  nymph  of  Perla  (Fig.  7f,  ) and  in 

Perlesta,  (Fig.  100)  , approximately  the  same  length  in  Capnia  (Fig. 

\ 

88),  and  shorter  than  the  galea  in  Isopteryx  (Fig.  80) , Taeniop- 
teryx (Fig.  82)  Xempra  (Fig.  84),  Pteronarcys  adult  (Fig.  yo) , 
nymph  (Fig.  91),  Chloraperla (Fig.  95),  and  Alloperla  (Fig.  97). 

The  palpi  of  the  nymphs  of  Perla  (Fig.  78)  and  Pteronar- 
cys(Fig.  9l),  and  of  Taeniopteryx  (Fig.  82)  are  shorter  than  the 
maxillae.  Each  palpus  consists  of  five  segments.  The  proximal  seg- 
ment is  always  short.  The  relative  length  of  the  different  seg- 
ments varies  according  to  the  species. 

The  parartis  is  well  developed  in  Nemura  (Fig.  86) , the 
nymph  of  Pteronarcys  (Fig.  92),  and  Capnia  (Fig.  87).  It  is  large 
and  chitinized.  Tne  exparartis  is  enlarged  and  articulates  with  a 
par,  coila  (Fig.  86),  The  premaxatendon  is  attached  to  the  entcparar 
tis  adjacent  to  the  alacardo  snd  the  maxacoria.  In  generalized 
stone-flies  the  premaxatendon  is  well  developed.  It  is  rudimentary 
in  the  specialized  species.  In  Taeniopteryx  (Fig.  82)  it  is 
short  but  dilated  while  in  most  of  the  other  species  it  is  longer 
and  slenderer. 


29 


The  maxacoria  (Fig.  82)  is  the  membrane  on  the  dorsal  as- 
pect attached  to  the  cardo,  the  stipes,  and  the  lacinia.  The 
labicoria  (Fig.  83)  is  the  membrane  on  the  ventral  aspect  which 
fuses  with  the  cardo,  the  subgalea,  and  the  lacinia. 

The  labium  is  the  appendage  of  th§  sixth  segment  of  the 
head,  a.nd  is  formed  by  the  fusion  of  a pair  of  appendages  on  the 
meson.  It  is  attached  to  the  cervicoria  and  to  the  ventral  as- 
pect of  each  maxilla  by  labicoria.  The  labium  is  typically  or- 
thopteran  in  form  in  the  Plecoptera.  The  submentum  is  the  large 
subquadrangular  sclerite  attached  to  the  cervicoria.  The  lateral 
edges  are  folded  under  and  are  continuous  with  the  labicoria.  The 
proximal  and  the  distal  ends  of  the  submentum  are  often  emarginate 
and  when  not  emarginate  are  usually  convex.  The  mentum  is  always 
small.  The  stipulae  are  the  sclerites  distad  of  the  mentum.  They 
bear  the  glossae  and  the  paraglossae  at  the  distal  end,  and  a pal- 
piger  on  either  edge.  Each  palpiger  is  a rounded  protuberance  at 
the  disto-lateral  or  lateral  aspect  of  a stipula  and  bears  a pal- 
pus. Each  palpiger  is  usually  separated  from  the  stipula  by  a 
suture  or  a furrow.  The  glcssae  are  the  two  mesal  projections  from, 
the  stipul  e.  The  fissure  separating  the  glossae  is  always  pres- 
ent in  the  Plecoptera.  It  is  the  mesarima.  Laterad  of  each 
glcssa  is  a paraglossa  which  is  usually  distinctly  separated  from 
the  glossa  by  a furrow,  the  latarima.  The  labia  of  the  Plecoptera 
a.re  of  two  types:  those  which  have  the  paraglossae  separated  from 
the  stipulae  by  a suture,  as  in  the  cockroach,  and  those  on  which 
the  paraglossae  are  indistinguishably  fused  with  the  stipulae.  The 
first  group  represents  the  most  generalized  condition,  the  latter 
the  specialized. 


N • I 

'N 


The  subme ntum  in  Acroneuria  (Pig,  //o)  is  broader  than 
long.  It  is  convex  on  the  proximal  end  and  concave  on  the  distal. 
The  same  condition 'is  found  in  the  nymph  of  Perla  (Fig. /d'*),  Per- 
lesta  (Fig.// 3 ) # Alloperla  (Fig. //&)  has  a submentum  which  is  slight 

V*  . 

ly  concave  at  both  ends  and  convex  on  the  sides.  It  is  slightly 

brrader  than  long,  Chloraperla  (Fig.  W)  hnd  Pteronarcys  adult 

(Fig,  /<$<?)  and  nymph  (Pig.  /dfif  show  the,  same  general  form.  In  Isop- 

teryx  (Pig,  /t*>)  and  TaeniOpteryx,  the  submentum  is  longer  than 

broad.  It  is  emarginate  at  both  ends.  In  Idopteryx  (Fig.  n*>)  the 

sides  at  the  proximal  end  are  roughly  parallel"  on  the  distal  half, 
toward  which 

they  converge  mm  ti  e distc  1 end.  /is  much  narrower  than  the  proximal. 
The  submentum  of  Leuctrgi  (Pig,  to*)  is  roughly  hexagonal  in  outline, 
in  ITemura  (Fi g./aC*)  and  Capnia  (Fig./7^.),  rectangular,  broader  than 

A 

long.  In  Capnia  (Fig.  //a.)  the  distal  margin  is  slightly  convex, 
the  proximal  concave. 

In  the  Plecoptera  the  mentum  is  usually  a narrow  sclerite 
closely  associated  with  the  submentum.  When  the  mentum  is  wanting 
it  is  eigher  fused  with  the  submentum  or  is  obsolete.  The  mentum  is 
a narrow  band  in,  Acroneuria  (Pig Mp  ),  nymph  of  Perla  (Fig.  /a')) , 
Alloperla  (Fig.  // (?)  , Capnia  (Fig.  //a)  , Ueirjira  (Fig.  /Ob)  , and 
Chloraperla  (Fig. y/V).  In  Chloraperla  it  is  concave,  in  Capnia, 
convex,  and  in  the  other  forms,  straight.  The  mentum  is  slightly 
triangular  in  shape  in  Pteronarcys,  adult  and  nymph  . (Figs.  /Oe7,/0£r')t 
being  widest  on  the  meson.  It  is  not  well  developed  in  Taeniopteryx 
(Fig.  ///)  , Perlesta  (Pig.  //3),  and  Isopteryx  (Fig.//<o).  The  stipu- 
lae  is  a narrow  area  in  the  nymph  of  Perla  (Fig.  /o')) . It  is  similar 
though  larger  in  Acroneuria  (Fig.  I/O),  It  is  longest  in  the  adult 


31 

of  Pteronarcys  (Fig./<J^). 

Each  palpiger  is  distinct  in  Pteronarcys  nymph  (Fig./0ff), 
and  adult  (Fij./^f),  Nemura  (Fi  ./^6),  Chlorape^a  (Fig. //^) , Isop- 

A 

teryx  (Fig. //<?“)  , Alloperla  (Fi ^,//6?) , Taeniopteryx  (Fig.  f//),  Cap- 

nia  (Fig.//^.)  , Leuctra  (Fig./Od*),  Peilesta  (Fig.//3  ) , and  Acrone  *i 

(Fig. //0).  In  the  nymph  of  Pe,rla  (Fig.  /07)  it  is  not  separated  "by 

a distinct  suture.  Tne  glossae  in  trie  most  generalized  forms  are 

divergent.  The  distal  end  is  bluntly  rounded.  In  the  nymph  of 

Perla  (Fig.  107)  they  are  short  and  broad,  triangular  in  shape. 

The  same  type  is  found  in  Acroneuria  (Figi//0  ),  Alloperla  (Fig. 

//£?  ) , Chloraperla  (Fig  .//£/  ),  Isopteryx  >(Fig  J f&  and  Perle  t 

(Fig.  1 1?>).  The  adult  of  Pteronarcys  (Fig,  /0*7  ),  Hemoura  (Fig.’ 

> 

Jd&),  and  Taeniopteryx  (Fig.  ill  ) have  glossae  intermediate  in 
form  between  the  two  types.  They  are  distinctly  separated  from 
each  other  by  the  mesarima  and  from  the  paraglossae  by  the  latarima. 
The  lateral  edge  is  as  long  as  the  mesal.  They  are  slightly  di- 
vergent. They  are  shorter  in  -Taeniopteryx  than  in  Pteronarcys.  In 
the  nymph  the  glossae  are  of  the  same  type  as  in  the  adult  but  are 
convergent  at  the  distal  end  and  bear  a thick  cluster  of  long 
setae.  The  glossae  in  Leuctra  (Fig.  (0$)  are  almost  adjacent  for 
their  entire  length  on  the  meson  and  in  Capnia  they  are  adjacent 
througnout  their  mesal  length. 

The  paraglossae  of  Acroneuria  (Fig.  //0) , Perlesta  (Fig. 
/h3  ),  Allopdrla  (Fig.  //k)  , Chloraperla  (Fig.  / r*y  ) , and  Isopteryx 
(Fi g.//p~  ) are  very  similar  in  shape,  they  converge  toward  the 
distal  end  and  are  roughly  triangular  in  outline.  The  nymph  of 
Perla  (Fig.  107)  has  paraglossae  more  nearly  globular  in  shape  than 
in  the  preceding  forms.  The  ventro.-lateral  edge  bears  a fringe  of 


32 


long  setae. 

The  paraglossae  of  Pteronarcys  adult  and  nymph.  (Figs. 

}.08,  109)  are  longer  and  narrower  than  in  the  preceding  forms.  In 
the  nymph  there  is  a row  of  distinct  3etae  on  the  lateral  aspect. 

e paraglOBsae  of  Leuctra  (Fig./tfCT)  are  not  separated  from  the 
stipulae  but  they  are  longer  and  the  latarima  is  distinct.  Their 
distal  eau  is  larger  than  the  proximal.  Lemur a (Fig.  106)  and 
Taeniopteryx  (Fig.  .Ill)  have  short  paraglossae  which  are  not 
separated  from  the  stipulae.  Capnia  (Fig.  112)  has  the  most 
specialized  paraglossae  figured.  They  are  in  close  proximity  to  the 
glossae  and  are  almost  uniform  in  width.  The  glossae  and  para- 
glossae are  the  same  length.  The  mesarima  is  the  fissure  which 
spparates  the  glossae.  In  the  nymph  of  Perla  (Fig.  107)  it  is  V- 
shaped,  broadly  rounded  at  the  base,  and  extends  almost  to  the  men- 
turo.  The  base  of  the  mesarima  is  wider  in  Acroneuria  (Fig.  110) 
than  in  the  nymph  of  Perla  but  it  is  comparatively  shorter.  In 
Perlesta  the  mesarima  is  narrow  at  the  base.  The  same  type  mesa- 
rima is  found  in  Alloperla  (Fig.  Il6)  , Isopteryx  (Fig,  115) » 
Chloraperla  (Fig.  114),  the  adult  of  Pteronarcys  (Fig,  109) , 

Fermi r a (Fig.  lOo) , and  Taeniopteryx  (Fig.  Ill),  but  it  is  much 
smaller.  In  the  nymph  of  Pteronarcys  (Fig.  108)  the  mesarima  is 
broader  at  the  base  than  at  the  distal  end. 

The  fundarima,  is  the  proximal  end  of  the  mesarima  when 
this  furrow  extends  between  the  stipulae,  when  the  glossae  are 
fused  it  is  the  line  which  marks  the  fusion.  It  is  present  as  such 
in  Leuctra  (Fig.  105) , and  in  Capnia  (Fig.  112) , where  the  glossae 
are  fused.  In  lie  mu  r a (Fig.  106)  and  Taeniopteryx  (Fig.  Ill  ) it 


* 

. 

. 


33 


marks  the  line  of  fusion  of  the  stipulae.  In  Capnia  (Fir.  112)  it 
is  obsolete  for  a short  distance  proximad  of  tne  glossae  but  appears 
between  the  stipulae  opposite  the  palpigers.  The  distal  end  of  tne 
fundarima  is  present  in  the  adult  of  Pteronarcys  (Fig.  109)  as  a 
large  dark  area. 

The  latarimae  are  distinct  in  the  nymphs  of  Perla  (Fig, 

1 °7),  Acroneuria  (Fig.  110),  Pterinarcys  nymph  (Fig.  108)  and  adult 
(Fig.  109),  Perlesta  (Fig.  113) , Chloraperla  (Fig.  114),  Isopteryx 
(Fig.  115),  Alloperla  (Fig.  llo) , Leuctra  (Fig.  105),  Nemoura  (Fig. 
106),  and  Taeniopteryx  (Fig.  111).  The  laterimae  appear  as  lines 
in  Capnia  (Fig.  112).  In  Taeniopteryx  (Fig.  Ill),  Leuctra  (Fig. 
105),  a^d  Uern^ira  (Fig,  106)  the  glossae  and  the  paraglossae  are 
adj  acent . 


SULIMARY 


34 


The  detailed  anatomy  of  the  head  of several  species  of 
Plecoptera  which  had  not  been  previously  studied  were  examined. 

The  parts  have  been  homologized  and  their  taxonomic  relations 
determined.  A comprehensive  and  representative  series  of  genera 
was  studied  to  determine  the  lines  of  development  and  specializa- 
tion within  the  order. 

Specialization  is  indicated  by  the  absence  of  sutures,  a 
reduction  in  size,  a change  in  form  or  position,  and  the  loss  of 
chitinization  of  sclerites  and  also  of  the  appendages.  There  is  a 
striking  difference  in  structure  between  the  most  generalized  and 
the  most  specialized.  The  former  are  distinctively  of  the  orthop- 
teran  type  but  a:  e alv/a..  s,  in  practically  every  particular,  more 
specialized  than  the  average  of  this  type,  as: 

(1)  The  epicraneal  suture  is  not  as  well  developed  in  the 
Plecoptera  as  it  is  in  most  of  tne  Orthoptera.  The  cephalic  por- 
tion of  the  epicraneal  arms  is  obsolete  in  all  of  the  species,  whi^e 
in  Capnia  and  Leuctra  the  stern  is  obsolete.  Both  the  stem  and  the 
arms  are  wanting  in  Isopteryx  and  Xernoura.  The  epicraneal  suture 

is  always  present  in  tne  Ortnop tera.  The  fronto-genal  sutures 
which  represent  the  cephalic  portions  of  the  epicraneal  amis  are 
never  present  in  the  Plecoptera. 

(2)  The  precoil?„e  which  are  always  exposed  on  the  dorsal 
or  cephalic  aspect  of  the  head  of  orthopterans  are  not  visible  from 
the  dorsal  aspect.  They  are  located  on  the  lateral  aspect  and  are 
always  concealed  by  the  dorsal  ledge. 

(3)  The  f rente- clypeal  suture  is  entirely  wanting  in 


35 

many  of  the  species  and  when  present  is  usually  supported  by  a lamel- 
la-like extension  of  the  pre  tentonae „ . 

(4)  The  occipital  foramen,  by  its  migration  dorsal  and 
caudad,  opening  on  the  caudal  aspect,  indicates  a greater  degree  of 
specialisation  than  is  found  in  any  orthopte  ans. 

(5)  The  occipital  foramen,  by  its  migration  caudad. and 
dor. ad  opening  on  the  caudal  aspect,  indicates  a greater  degree 

of  specialization  than  is  found  in  any  orthopteran . The  contour  of 
that  portion  of  the  iiead  bearing  the  paracoila  awi  the  postcoila 
and  bounding  the  maxillae  and  labium,  is  characteristic  of  those 
specialized  insects  that  have  the  mouth  at  one  end  of  the  head  and 
the  occipital  foramen  at  the  other. 

(6)  The  an  tennaria  is  as  small  and  of  the  same  degree  of 
development  in  Capnia  as  in  the  Orthoptera.  In  Pteronarcys  it  is 
large  but  the  increase  in  size  seems  to  indicate  specialization 
rather  than  generalization  as  it  is  smaller  in  the  nymph  than  in  the 
adult,  Tnis  is  paralleled  in  the  adult  by  the  much  greater  size  and 
use  of  t:.e  antennae. 

(7 ) The  mandibles  of  the  generalized  soecies  and  of  the 
nymphs  are  typically  orthopteran  in  form.  They  are  large,  have  well 
develop  d dentes,  and  are  heavily  chitinized.  In  the  specialized 
forms  the  mandibles  are  strikingly  different.  They  are  smaller, 

the  dentes  are  usually  reduced  or  absent  , as  in  the  adult  of 
Pteronarcys,  and  are  poorly  chitinized. 

(8)  The  orthopteran  type  of  maxillae  is  characteristic 
of  the  generalized  Plecoptera.  Great  reduction  in  size,  form,  fu- 
sion of  parts  and  degee  of  chitinization  has  taken  place  in  the 


s 


. 

■' 

- 

' 

. 


36 


specialized  species.  The  proxagalea  and  the  distagalea  are  inc 
tinguishably  fused,  and  the  suture  "between  the  subgalea  and  the  f. 
stipes  is  obsolete.  The  dardo  is  not  separated  from  the  stipes 
a distinct  suture,  and  tne  line  separating  its  two  parts  can  not  be 
distinguished.  The  parartis  in  Acroneuria  is  rudimentary  or 
wanting,  and  the  cuticle  of  the  proximal  portion  of  the  maxillae  is 
continuous  with  that  of  the  head-capsule. 

(9)  The  labia  is  also  typically  orthopteran  in  form. 

whi  c h 

This  is  true  of  the  speciejs/in  other  respects  are  distinctly 
specialized,  while  in  those  that  ordinarily  are  most  specialized 
have  the  most  generalized  labia.  This  is  paralleled  in  the  Hepiali- 
dae,  which  xe  the  most  generalized  Lepidoptera  in  practically  all 
of  their  structures,  yet  have  been  specialized  by  the  loss  of 
mouth-parts . 

The  Plecoptera  has  been  regarded  by  many  as  a more 
generalized  group  of  insects  than  the  Orthoptera.  This  may  be 
true  in  some  respects  but  as  an  order  the  structure  of  the  head 
and  mouth-parts  shows  a greater  degree  of  specialization  than  do 
tne  orthopterans  and  many  of  tne  most  specialized  species  of 
stone-flies  show  specializations  that  are  more  striking  than  those 
found  in  any  of  the  Orthoptera., 


PLATS  I 


Explanation  of  Figures 

Dorsal  Aspect  of  the  Head 

1.  Leuctra  klapaleki 

2.  Hemoura  varigata 

3.  Acroneuria  abnormis 

4.  Taeniopteryx  frigida 

5.  Isopteryx  tripunctata 

6.  Perla  immarginata,  nymph 

7.  Alloperla  minuta 

8.  Pteronarcys  rdgalis,  adult 

9.  Pteronarcys  regalis,  nymph 

10.  Capnia  necydaldides 

11.  Chloraperla  grammatica 

12.  Perlesta  placida 


PLATE  I 


PLATE  II 


Explanation  of  Figures 

Ventral  Aspect  with.  Mouth-parts  in  place 

13.  Leuctra  klapaleki, 

14 . ITemouro.  vari  gata . 

15.  Taeniopteryx  frigida. 
l6 Acroneuria  atnomis, 

17.  Pteronarcys  regalis,  adult. 

18.  Pteronarcys  regalis,  nymph. 

19.  Perla  irnmarginata,  nymph. 

20.  Ohio rape rla  grammati ca, 

21.  Capnia  necydaloides, 

22.  Isopteryx  tripunctata. 

23.  Perlesta  placida. 

24.  All  ope  rla.  rninuta. 


PLATE  II 


39 


PLATE  III 

Explanation  of  Figures 

Ventral  Aspect  with  Mouth-parts  Removed 
25.  Leuctra  klapaleki. 

2o.  Ner.iura  varigata. 

27.  Acroneuria  ahnormis. 

28.  Pteronarcys  regalis,  nymph. 

29.  Pteronarcys  regalis,  adult. 

30.  Capnia  necydaloides. 

31.  Perla  immarginata,  nymph. 

32.  Taeniopteryx  frigida. 

33.  Chloroperla  grammatica. 

34.  Isopteryx  tripunctata, 

35.  Perlesta  placida. 

36.  Alloperla  minuta. 


PLATE  IV 


Explanation  of  Figures 
Hand i Lies 

37.  Pteronarcys  regalis  nymph,  dorsal. 
3c.  Pteronarcys  regalis  nymph,  ventral. 
39.  Pteronarcys  regalis  nymph,  lateral. 
4-0.  Nerijura  varigata,  ventral. 

4-1.  Neri^ira  varigata,  dorsal. 

42.  ITenfiira  varigata,  lateral. 

43.  Pteronarcys  regalis,  adult,  ventral. 

44.  Pteronarcys  regalis,  adult,  dorsal. 

45.  Pteronarcys  regalis,  adult,  rnesal. 

46.  Taeniopteryx  frigida,  dorsal. 

47.  Taeniopteryx  frigida,  ventral. 

48.  Taeniopteryx  frigida,  lateral. 

49.  Leuctra  klapalekt,  dorsal. 

50.  Leuctra  klapalekt,  ventral. 

7l.  Leuctra  klapalekt,  lateral. 

Oapmia  necydaloides , ventral. 

53.  Vapnia  necydaloides,  dorsal. 

54.  Capnia  necydaloides , me sal. 

55.  Capnia  necydaloides,  lateral. 

56.  Perla  immarginata  nymph,  ventral. 

. Perla  immarginata  nymph,  dorsal. 

5c.  Perla  immarginata  nymph,  mesal. 


41 


PLATE  IV  (Continued) 

59.  Isopteryx  tripunctata,  ventral. 

60.  Isopteryx  tripunctata,  dorsal. 

61.  Isopteryx  tripunctata,  ental. 

62.  Acroneuria  abnormis , 'dorsal. 

63.  Acroneuria  abnormis , Vventral . 

64.  Acroneuria  abnormi s , V ental . 

65.  Acroneuria.  abnormis,  ventral. 

66.  Acroneuria  abnormis,  meso-dorsal . 

67.  Alloperla  minuta,  dorsal. 

6.8.  Alloperla  minuta,  ventral. 

69.  Alloperla  minuta,  ental. 

70.  Chloraperla  grammatica,  dorsal. 

71.  Chloraperla  grammatica,  ventral. 
78.  Chloraperla.  grammatica,  lateral. 

73.  Perlesta  placida,  dorsal. 

74.  Perlesta  placida,  ventral. 

75.  Perlesta  placida,  lateral. 


PLATE  IV 


PLATE  V 


Explanation  of  Figures 
Maxillae 

76.  Leuctra  klapaleld,  dorsal. 

77.  Leuctra  klapaleki,  ventre 1. 

78.  Perla  iramarginata,  nymph,  dorsal. 

79.  Perla  immarginata,  nymph,  ventral. 

80.  Isopteryx  tripunctata,  dorsal. 

81.  Isopteryx  tripunctata,  ventral. 

82.  Taeniopteryx  frigida,  dorsal. 

83.  Taeniopteryx  frigida,  ventral. 

84.  Nenura  varigata,  dorsal. 

85.  ITemura  varigata,  ventral. 

86.  Lemur a varigata, 

87.  Capnia  necydaloides , dorsal. 

88.  Capnia  necydaloides,  ventral. 

89.  Pteronarcys  regalis  adult,  dorsal. 

90.  Pteronarcys  regalis  adult,  ventral 

91.  Pteronarcys  regalis  nymph,  ventral 

92.  Pteronarcys  regalis  nymph , dorsal. 

93.  Acroneuria  abnormis,  dorsal. 

94.  Acroneuria  abnormis,  ventral. 

95.  Chloraperla  grammatica,  ventral, 

96.  Chloraperla  grammatica,  dorsal. 

97.  Alloperla  minuta,  ventral. 

98.  Alloperla  minuta,  dorsal. 


PLATE  V (Continued) 


99.  Perlesta  placida,  dorsal. 


100. 

Perlesta  placida,  ventral. 

Antennae 

101. 

Pteronarcys 

recalls , 

nymph , 

ventral 

aspect. 

102. 

Pteronarcys 

regalis, 

adul t , 

dorsal  aspect. 

103. 

Pteronarcys 

regal is , 

nymph, 

dorsal 

aspect. 

104. 

Pteronarcys 

rogalis, 

adult , 

ventral 

aspect . 

PLATE  V 


44 


PLATE  VI 

Explanation  of  Figures 


Labia 


105. 

Leuctra  klapalekc. 

106. 

Nemoura  varigata. 

107. 

Perla  immarginata , nymph. 

• 

00 

0 
1 — 1 

Pteronarcys  regalis,  nymph 

109. 

Pteronarcys  regalis,  adult 

no. 

Acroneuria  abnormis. 

■111. 

Taeniopteryx  frigida. 

112. 

Gapnia  necydaloides . 

113. 

Perlesta  plucida.  * 

114. 

Chi 0 rap  e rl a gramma t i ca . 

115. 

Isopteryx  tripunctata. 

116. 

Alloperla  minuta. 

